A standard column or prop used in a mine to hold up the ceiling or roof comprises a pair of aluminum or steel tubular tubes that telescope in each other. The outer end of one of the tube is braced against the ceiling surface and the outer end of the other tube is braced against the floor surface while the inner ends fit within each other. As described in U.S. Pat. Nos. 3,737,134 and 4,185,940 respectively of Foon and Spies the upper end of the outer tube is frustoconically upwardly flared and annular wedges engage between this flared region and the inner tube. These wedges allow the tubes to be moved axially apart but resist movement together as the wedges bite into the inner tube. The wedges permit some downward movement of the inner tube in the outer tube, giving the column its vertical resiliency. The upper end of the inner tube is formed as a nut into which is threaded a massive bolt whose upper end bears via a rigid dished plate on the ceiling, this plate having no significant resiliency. The setting force, that is the vertical load the column is set to bear, is established by screwing this bolt out of the column against the ceiling.
The wedges allow the prop to be compressed vertically to a limited extent if the ceiling or floor shifts. Thus for a slight shift the prop will not collapse but will remain in place. Once the column has shortened somewhat, it becomes solid and resists any further shortening. Thus such a prop acts like a pressure-relief valve.
With such systems it is fairly difficult to set the starting post length, which determines the setting force, accurately and easily. Normally the parts that establish this post length (and the setting force) are independent of the elements that provide for resiliently resisted telescoping together. Thus the known props are excessively complex and expensive, which is particularly disadvantageous since they are normally set and left, that is they cannot easily be reused.